Mitochondrial Ca(2+) Uptake Relieves Palmitate-Induced Cytosolic Ca(2+) Overload in MIN6 Cells

Saturated fatty acids contribute to β-cell dysfunction in the onset of type 2 diabetes mellitus. Cellular responses to lipotoxicity include oxidative stress, endoplasmic reticulum (ER) stress, and blockage of autophagy. Palmitate induces ER Ca(2+) depletion followed by notable store-operated Ca(2+) entry. Subsequent elevation of cytosolic Ca(2+) can activate undesirable signaling pathways culminating in cell death. Mitochondrial Ca(2+) uniporter (MCU) is the major route for Ca(2+) uptake into the matrix and couples metabolism with insulin secretion. However, it has been unclear whether mitochondrial Ca(2+) uptake plays a protective role or contributes to lipotoxicity. Here, we observed palmitate upregulated MCU protein expression in a mouse clonal β-cell, MIN6, under normal glucose, but not high glucose medium. Palmitate elevated baseline cytosolic Ca(2+) concentration ([Ca(2+)](i)) and reduced depolarization-triggered Ca(2+) influx likely due to the inactivation of voltage-gated Ca(2+) channels (VGCCs). Targeted reduction of MCU expression using RNA interference abolished mitochondrial superoxide production but exacerbated palmitate-induced [Ca(2+)](i) overload. Consequently, MCU knockdown aggravated blockage of autophagic degradation. In contrast, co-treatment with verapamil, a VGCC inhibitor, prevented palmitate-induced basal [Ca(2+)](i) elevation and defective [Ca(2+)](i) transients. Extracellular Ca(2+) chelation as well as VGCC inhibitors effectively rescued autophagy defects and cytotoxicity. These observations suggest enhanced mitochondrial Ca(2+) uptake via MCU upregulation is a mechanism by which pancreatic β-cells are able to alleviate cytosolic Ca(2+) overload and its detrimental consequences.